Compact fluid motor control system with float position

ABSTRACT

A control system for a fluid motor includes a motor direction control valve having a spool and means for selectively shifting the spool in response to fluid pressure signals from a manually operated pilot valve. The directional valve has make-up valve means which automatically opens a bypass passage between the motor ports and fluid return passage when necessary to prevent cavitation of the motor due to an overrunning condition from external load force. The spool of the pilot valve has four positions including one for each direction of motor motion, another for blocking any motor motion and a float position for deliberately interconnecting both motor ports and the fluid return passage so that the motor may move in either direction as determined by external load forces. Unlike the manually actuated pilot valve, the spool of the larger and more costly directional valve need have only three positions as the float function is achieved by relieving the fluid pressure which normally biases one of the make-up valves towards the closed position. Thus a substantial reduction of size and cost of the direction control valve is realized.

g [22] Filed:

[ COMPACT FLUID MOTOR CONTROL SYSTEM WITH FLOAT POSITION -[52] US. Cl ..137/625.63,l37/596.l2,137159615 [51] Int. Cl. Fl6k 31/12, F16k 3l/l 4 F16k 31/36 [58] Field of Search 137/62563, 596.2, 596.15, l37/596.12, 596.13, 596.14

[56] References Cited UNITED STATES PATENTS 2,981,287 4/1961 Caslow l37/625.63

Primary Examiner-Henry T. Klinksiek Assistant Examiner- -Robert J. Miller Attorney, Agent, or Firm-Phillips, Moore, Weissenberger, Lempio & Strabala [57] ABSTRACT A control system for a fluid motor includes a motor direction control valve having a spool and means for selectively shifting the spool in response to fluid pressure signals from a manually operated pilot valve. The directional valve has make-up valve means which automatically opens a bypass passage between the motor ports and fluid return passage when necessary to prevent cavitation of the motor due to an overrunning condition from external load force. The spool of the pilot valve has four positions including one for each direction of motor motion, another for blocking any motor motion and a float position for deliberately interconnecting both motor ports and the fluid return passage so that the motor may move in either direction as determined by external load forces. Unlike the manually actuated pilot valve, the spool of the larger and more costly directional valve need have only three positions as the float function is achieved by relieving the fluid pressure which normally biases one of the make-up valves towards the closed position. Thus a substantial reduction of size and cost of the direction control valve is realized.

8 Claims, 4 Drawing Figures PATENTED 81974 sumaarz COMPACT FLUID MOTOR CONTROL SYSTE WITH FLOAT POSITION BACKGROUND OF THE INVENTION This invention relates to fluid-driven motors and more particularly to control. systems therefor which enable an operator to select any of several distinct operating modes of a motor.

Fluid-driven motors are usually controlled with a directional control valve wherein a spool or other valve member is shiftable between a Hold position at which the motor is stopped and another position for operating the motor in a first direction and still another position for reversing operation of the motor. In many instances, the motor moves a load which may on occasion tend to move faster than is provided for by the rate at which fluid is supplied to the motor through the directional control valve.

For example, powered loaders of the type described in US. Pat. No. 3,599,813 and used to handle loose earth or other bulk material have a bucket carried on a pair of pivoting lift arms at the front of the vehicle wherein upward and downward movement of the bucket is typically provided for by fluid motors controlled by the vehicle operator. When the loaded bucket is being lowered, gravitational force may tend to drive the fluid motors faster than is provided for by the supply of driving fluid. Under this condition, motor cavitation will occur, with well known undesirable effects, unless corrective means are provided. One such means of preventing cavitation are make-up valves which sense incipient cavitationand open to supplement the driving fluid flow to the motor with fluid:

which is being discharged from the motor.

The action of these conventional make-up valves is automatic and not subject to operator control. In some instances, it may be desirable to deliberately cause the motor to respond to the load rather than to the rate at which driving fluid is received from the pump. For example, in the loader discussed above, considerable time and power savings may be realized by allowing the loaded bucket to drop under the influence of gravity faster than is provided for by the maximum rate at which driving fluid can be supplied to the motor.

This is conventionally provided for by a fourth position of the directional control valve known as the Float position. The Float mode of operation is realized by manually initiating communication of the two ports of the motor with each other and with the return line to tank.

Providing of a Float position in the control system may also be. useful for purposes other than utilizing gravity force to speed movement of the load. For example, a loader as discussed above is often used to pick up loose material by pushing the bucket long the surface of the ground to receive such material. On uneven terrain, it is desirable that the bucket follow the contour of the ground and a Float position of the control system enables the bucket to do this.

Heretofore, a Float condition in fluid motor control systems has been provided for by means of directional control valve complications such as additional lands on the principal spool and appropriate passages in the valve body additional to those needed for establishing the other modes of motor operation discussed above. This adds substantially to the length of the directional control valve spool and the complexity of the valve body. Directional control valves are often a necessarily large assembly and these conventional provisions for a Float position add to the costs of such valves and substantially increase the space requirements for positioning the control system in a working context.

SUMMARY OF THE INVENTION This invention is a compact and economically manufactured system for controlling a fluid motor wherein a Float mode of operation is to be provided for and wherein make-up valves are present to forestall cavitation. Rather than extending and complicating the directional control valve spool and body to provide for a Float position, the invention utilizes the make-up valve means for this purpose. In particular, manual shifting of the operators pilot valve, which controls the directional valve, to the Float setting is arranged to relieve the pressure which normally biases a make-up valve towards a closed condition. The make-up valve may then open to intercommunicate both ports of the motorto establish the Float condition.

Providing for the above-described four modes of motor operation by use of a directional control valve spool or the like which need have only three positions for this purpose results in a smaller, less costly and more efficient control system.

Accordingly, it is an object of this invention to provide a more compact and economical system for controlling a fluid motor wherein it is desired to provide for a Float condition in addition to a Hold condition and drive conditions providing for motor operation in either direction.

The invention, together with further objects and advantages thereof, will best be understood by reference to the following description of a preferred embodiment taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 illustrates a fluid motor and control system therefor in accordance with the invention wherein valve bodies are shown in section and certain other components are shown schematically, the system being shown in the Hold or neutral position,

FIG. 2 is a section view of the pilot valve portion of FIG. 1 shown shifted to an Extend position at which the motor is caused to operate in a first direction,

FIG. 3 is a view of the pilot valve shown shifted to a Contract position at which the motor is caused to operate in an opposite direction, and

FIG. 4 is a section view showing the pilot valve shifted to the Float position.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawings and more particularly to FIG. 1, a linear fluid motor 10 which is controlled by the present invention may be of conventional design and is therefore shown schematically in the drawing. A pair of such motors 10 may, for example, be used to raise and lower the bucket of a loader vehicle of the kind previously discussed; however, it should be appreciated that the present control system is not limited to this particular usage nor is it limited to fluid motors of the linear kind, but is also applicable to rotary motors.

A linear motor of this form has two fluid ports 11 and 12 to which actuating fluid may be supplied. If actuating fluid is supplied to one such port 11, the motor retracts and fluid is discharged from the other port 12. If actuating fluid is supplied through port 12, the motor extends with fluid being discharged through the other port 11.

Pressurized fluid for driving the motor 10 is supplied by a pump 13 which draws fluid from a reservoir 14. The outlet of pump 13 is communicated to an inlet port 16 of a directional control valve 17 by way of a conduit 15. A relief valve 18 is connected between conduit and reservoir 14 to establish a maximum fluid pressure which can be transmitted to the motor and to return excess fluid directly to the reservoir. Directional valve 17 has a first valve member or spool 19 located in a first bore 21 of a valve body 22 and the spool is movable axially to one of three positions which respectively constitute a Hold position at which motor 10 is stopped, a Retract position at which the motor is driven in the direction R indicated by arrow 23, and a third or Extend position at which the motor is driven in the opposite direction E as indicated by arrow 23.

Spool 19 of valve 17 is urged toward a centered position in bore 21, which constitutes the Hold position, by a pair of springs 24 and 24a situated in bore 21 at opposite ends of the spool. Fluid inlet port 16 communicates by means of a passage 59 with an annular groove 61 at the center of bore 21. Bore 21 has an additional groove 71 spaced to one side of input groove 61 and which is communicated with motor port 11 through a valve body passage 69, and conduit 67. Similarly, bore 21 has still another groove 63, on the opposite side of central input groove 61, that is communicated with the motor port 12 through a valve body passage 64 and conduit 66. Bore 21 has still additional grooves 73 and 77 spaced outwardly from grooves 71 and 63 respectively, which are each communicated with the reservoir 14 by a drain passage 74 in the valve body and a drain conduit 76.

Spool 19 has a pair of grooves 62 and 72 situated on opposite sides of a central land 19' of the spool. Accordingly, when the spool 19 is at the above-described centered or Hold position, land 19' blocks driving fluid from both motor ports 11 and 12 while the end portions of the spool block communication between the two motor ports and drain passage 74. Thus, motor 10 is stopped and can neither extend nor retract. lf spool 19 is shifted to the left as viewed in FIG. 1, spool groove 62 communicates input port 16 with motor port 12 while spool groove 72 communicates motor port 11 with drain conduit 76 causing motor 10 to extend. Similarly, if spool 19 is shifted in the opposite direction, spool groove 72 transmits driving fluid from input port 16 to motor port 11 while groove 62 vents motor port 12 to drain causing the motor to retract.

Spool 19 is shifted between the above-described three positions for the purpose of controlling motor 10 in response to fluid pressure signals received through a manually controlled pilot valve 25. Pilot valve 25 has a body with a bore 32 in which a spool 31 is disposed for axial movement by the operator between four positions which are the Extend, Hold, Retract and Float positions. Pilot valve 25 receives pressurized fluid from a second pump 26 which draws fluid from reservoir 14 and transmits the flow to an input port 28 of the pilot valve through a conduit 27. A second relief valve 29 is connected between conduit 27 and reservoir 14 to establish the basic pilot fluid pressure and to return excess output of pump 26 to reservoir 14.

A valve body passage 56 communicates the input port 28 of pilot valve 25 with an annular input groove 58 at bore 32. Additional grooves 34 and 43 of bore 32 are situated at opposite sides of groove 58 and connect with output passages 33 and 42 respectively of the pilot valve. Conduits 49 and 53 respectively connect output passages 33 and 42 with ports 51 and 54 respectively at opposite ends of bore 21 of the directional control valve 17.

Bore 32 of the pilot valve has additional grooves 36 and 45 spaced outwardly from grooves 34 and 43 respectively. Groove 36 communicates with the reservoir 14 through a drain passage 37 while groove 45 connects with reservoir 14 through a passage 46. Spool 31 has spaced-apart grooves 35 and 44 which in the Hold position of the spool communicate grooves 34 and 43 with drain grooves 36 and 45 respectively while the intervening portion of the spool blocks flow between input groove 58 and either of grooves 34 and 43. Thus, at the Hold position of pilot valve 25, spring chambers 52 and 55 of directional control valve 17 are vented and the directional control valve assumes the Hold position at which motor 10 is stopped.

When the operator shifts the spool 31 of pilot valve 25 to the Extend position as depicted in FIG. 2, pilot pressure from inlet port 28 is transmitted through grooves 58, 44 and 43 and passage 42 to conduit 53 while conduit 49 is opened to reservoir 14 through passage 33, grooves 34, 35 and 36 and passage 37. Referring again to FIG. 1, this pressurizes spring chamber 55 of the directional control valve 17 while venting spring chamber 52. Directional control valve spool 19 is thereby shifted to the hereinbefore described position at which motor 10 is caused to extend.

Manual shifting of the spool 31 of pilot valve 25 to the Retract position depicted in FIG. 3 transmits pilot pressure from inlet groove 58 through grooves 35, 34 and passage 33 to conduit 49 while conduit 53 is communicated with reservoir 14 through passage 42 and grooves 43, 44 and 45. Referring again to FIG. 1, this action pressurizes spring chamber 52 of the directional control valve 17 while venting spring chamber 55. This shifts directional control valve spool 19 to the hereinbefore described position at which motor 10 is caused to retract.

Considering now means by which cavitation of motor 10 is forestalled, directional control valve 17 has a pair of cylindrical make-up valve members 78 and 79 disposed in additional bores 83 and 83a respectively of valve body 22 for axial movement therein. A spring 86 urges valve member 78 against a seat 81 through which bore 83 connects with a short passage to groove 73 and thus with drain passage 74. Similarly, a spring 86a in bore 83a urges valve member 79 against a seat 82 through which the bore connects with a passage 75a to groove 77 and drain passage 74. Accordingly, bores 83 and 83a respectively are normally blocked from communication with the drain passages by the valve members 78 and 79 but movement of either such valve member away from the associated seat 81 or 82 communicates the associated bore and motor port with drain passage 74.

An orifice 85 in the side of valve member 78 adjacent a shoulder 87 thereon communicates spring chamber fice 102 in valve member 79 adjacent a similar shoulder 103 thereof, transmits the fluid pressure of passage 9 64 to spring chamber 88. As the area of valve members 78 and 79 which is exposed to the pressure within spring chambers 84 and 88 is greater than the area of shoulders 87 and 103 on which the pressure acts oppositely, the valve members normally remain closed in the presence of pressure rises in the associated flow passages 69 and 64 except under certain conditions to be hereinafter described.

As previously described, during the Retract mode of motor operation, driving fluid is directed to port 11 of motor 10. Under this condition, directional control valve passage 69 is pressurized while passage 64 is communicated with the reservoir 14. If at this time external load forces on motor 10 cause the motor to Retract faster than is provided for by the flow of driving fluid into the motor 10 through passage 69, a pressure drop is produced therein which is communicated to spring chamber 84 through an orifice 85 in valve member 78. Simultaneously, a pressure rise is produced in passage 64 which is transmitted to passage 75 through drain passage 74. Accordingly, valve member 78 is lifted away from the associated seat 81 against the force of spring 86. Althrough passage 75 is connected to reservoir 14, the flow conduit 76 therebetween constitutes a sufficient flow restriction that adequate pressure is present in passage 75 to lift the valve member 78 from its seat once the pressure within spring chamber 84 has dropped sufficiently due to the incipient cavitation of motor 10.

When valve member 78 lifts from seat 81 as described above, the supply of driving fluid passing to motor port 11 through passage 69 is supplemented by additional fluid from drain passage 74 which fluid is the discharge flow from the other side of motor 10. Thus the end effect of valve member 78 is to provide for a direct exchange of fluid between the two ports of motor 10 when necessary to forestall cavitation.

The other make-up valve member 79 opens automatically to forestall cavitation during the Extend movement of motor 10 in a manner essentially similar to that described above for the make-up valve member 78.

In order to provide an additional mode of operation of the system wherein the motor 10 may move in either direction as determined by an external load force thereon, generally termed the Float position, conventional systems provide for an additional position of the directional valve spool 19 which requires additional grooves on the spool and in the bore 21 and additional valve passages in body 22. The present invention avoids this increase in the complexity, size and cost of the directional control valve assembly 17 by providing structure which enables the fluid pressure bias which normally tends to hold the make-up valve member 78 closed to be selectively relieved in response to manual movement of the pilot control valve spool 31 to the Float setting. In the absence of this fluid pressure bias in spring chamber 84, the make-up valve member 78 may lift from the associated seat 81 to provide for an exchange of fluid between ports 11 and 12 of the motor in response to an external load force acting thereon.

Considering now suitable structure for this purpose, if the pilot control valve 25 is shifted to the Float position as shown in FIG. 4, pilot pressure from inlet port 28 is transmitted to directional control valve 17 in the same manner as was previously described for the Retract position of the pilot valve which is closely adjacent the Float position. Referring again to FIG. I, pressure from inlet port 28 is transmitted to spring chamber 52 of directional control valve 17 through conduit 49 causing the spool 19' of the control valve to shift to the position at which driving fluid is supplied to the motor port 11 while motor port 12 is communicated with reservoir 14. However, unlike the Retract position, the Float position of pilot valve 25 communicates an additional groove 91 in pilot valve bore 32 with drain passage 37 through an additional groove 89 on pilot valve spool 31.

A passage 92 and conduit 94 connect groove 91 with a float valve 93 which is in turn connected to spring chambers 84 and 88 through separate conduits 95 and 99 respectively. Float valve 93 is a two position valve having a body 104 and internal spool 106 biased by a spring 107 towards a position at which conduit 94 is communicated with conduit 99 while being blocked from conduit 95. A pilot line 108 transmits fluid pressure from groove 58 of manual pilot valve 25 to one end of valve 93 to shift spool 106 to an alternate position whenever pump 26 is supplying fluid pressure. At this alternate position, depicted in FIG. 4, conduit 95 is communicated with conduit 94 while conduit 99 is blocked.

Referring now to FIGS. 1 and 4 in conjunction, it may be seen that with manual pilot valve 25 shifted to the Float position and with float valve 93 held at the alternate position by pilot pressure from pump 26, spring chamber 84 is vented to drain through conduit 95, valve 93, conduit 94, grooves 91 and 36 and passage 37. This enables the make-up valve member 78 to be lifted off the associated seat 81 by pressure in passage 69 which acts against shoulder 87 or in response to pressure in passage received through drain passage 74.

In effect this interconnects directional control valve inlet port 16, drain passage 74 and both of motor ports 11 and 12 to establish the desired Float conditions of the system.

It may be observed that achieving the Float position as described above requires that directional control valve spool 19' be shifted against spring 24a by pilot fluid pressure from pilot valve 25 as produced by pump 26. If the pilot pressure supply should fail for any reason, such as a breakdown of the engine which drives pump 26, the Float condition cannot be realized in the particular manner described above. However, a Float condition for one direction of motor movement can be highly desirable under this circumstance in order to enable a load supported by motor 10 to be lowered by gravity. The above described system inherently provides for realizing the Float condition for motor retraction in the event of pilot pressure failure, but in a somewhat different manner.

In particular, realization of the Float position as hereinbefore described requires venting of the pressure hehind only a single one of the valve members 78 and 79, valve member 78 in this example, since spool 19 is shifted at that time by pilot pressure to effectively bypass the other make-up valve 79. However, if pilot pressure has failed, spool 19 is spring centered and a float condition for retracting can only be achieved by venting the spring chambers behind the other make-up valve member 79.

This occurs automatically since in the absence of pilot pressure, float valve 93 is spring shifted to the position depicted in FIG. 1 at which conduit 99 is communicated with pilot valve groove 91. If the pilot valve 25 is then'shifted to the Float setting shown in FIG. 4, spring chamber 88 of FIG. 1 is vented to enable the motor 10 to retract under the influence of an external load acting thereon.

While the invention has been described with reference to a single preferred embodiment, many variations are possible and it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. In a control system for a fluid motor (10) which has first and second motor ports (11, 12) wherein admission of pressurized fluid to said first motor port causes motor operation in one direction and admission of pressurized fluid to said second motor port causes motor operation in the opposite direction, the combination comprising:

a directional control valve (17) having an inlet passage (59) for receiving pressurized fluid and having a discharge passage (74) and first and second outlet passages (69, 64) for connection to said first and second motor ports respectively and having a valve member (19) movable between a first motor operating position at which fluid from said inlet is transmitted to said first outlet while said second outlet is communicated with said discharge passage and a second motor operating position at which fluid from said inlet is transmitted to said second outlet while said first outlet is communicated with said discharge passage, and further having pilot means (51, 54) for shifting said valve member between said positions in response to fluid pilot pressure signals,

make-up valve means disposed in a bypass passage (75) between at least one of said outlet passages of said directional control valve and said discharge passage and having a first make-up valve member (78) movable to close said bypass passage and having means forming a first chamber (84) wherein fluid pressure derived from said one outlet passage acts against said first make-up valve member to urge said first make-up valve member towards the position at which said bypass passage is closed, and

pilot valve (25) having an inlet passage (56) for receiving pressurized fluid and a discharge passage (37) and an outlet means (33) communicated with said pilot means of said directional control valve for transmitting said fluid pilot pressure signals thereto to shift said valve member of said directional control valve, said pilot valve further having a valve member (31) movable between a plurality of motor operating positions for selectively transmitting said pilot pressure signals to said directional control valve, and being further movable to a float position, said pilot valve having venting means (36, 91, 95) for communicating said first chamber of said make-up valve means with said pilot valve means discharge passage upon said movement of said pilot valve member to said float position.

2. The combination defined in claim 1 wherein said first make-up valve member has a flow orifice (85) therethrough to transmit said fluid pressure of said one outlet passage of said directional control valve to said first chamber.

3. The combination defined in claim 1 wherein said first make-up valve member has a shoulder (87) thereon exposed to the fluid pressure of said one outlet passage.

4. The combination defined in claim 1 wherein said pilot valve means has means (35, 49) for transmitting said pressure signals to said directional control valve pilot means to shift said valve member of said directional control valve to one of said motor operating positions thereof while said pilot valve member is at said float position thereof and said pilot valve means is communicating said first chamber of said directional control valve with said discharge passage of said pilot valve means.

5. The combination defined in claim 1 wherein said make-up valve means further comprises means forming a second bypass flow passage (a) between the other outlet passage (64) of said directional control valve and said discharge passage thereof, a second make-up valve member (79) movable to close said second bypass passage, means forming a second chamber (88) wherein fluid pressure derived from said other outlet passage acts against said second make-up valve member to urge said second make-up valve member to close said second bypass passage, a float valve (93) connected between said second chamber and said venting means, and means for opening said blocker valve in response to an unpressurized condition in said outlet means of said pilot valve means whereby said pilot valve means communicates said second chamber to said pilot valve discharge passage if said pilot valve member is at said float position and said fluid pilot pressure signal is absent.

6. The combination defined in claim 5 wherein said float valve is connected between said venting means and both said first and second chambers and is spring biased towards a first position wherein said second chamber is communicated with said venting means while said first chamber is blocked therefrom, further comprising pilot means responsive to said fluid pilot pressure to urge said float valve to a second position at which said first chamber is communicated with said venting means while said second chamber is blocked therefrom.

7. In a control system for a fluid motor having a first port to which pressurized fluid is supplied to drive said motor in a first direction and having a second port to which pressurized fluid is supplied to drive said motor in an opposite direction, the combination comprising:

a directional control valve having a valve body with an inlet passage for receiving the pressurized fluid and first and second outlet passages for connection to said first and second motor ports respectively and a discharge passage for discharging fluid, said valve body having a bore therein, said directional control valve having a first spool movable in said bore between a centered Hold position at which both of said outlet passages are blocked from said inlet passage and said discharge passage and having a first operating position at which said inlet passage is communicated with said first outlet passage while said second outlet passage is communicated to said discharge passage and having a second motor operating position at which said inlet passage is communicated with said second outlet passage while said first outlet passage is communicated with said discharge passage, and having spring means for urging said first spool towards said Hold position thereof; make-up valve means having means forming a first bypass passage between said first outlet passage and said discharge passage and having means forming a second bypass passage between said second outlet passage and said discharge passage and having first and" second make-up valve members disposed in said first and second bypass passages respectively'each being movable to close the associated bypass passage, and having means defining a first chamber communicated with said first outlet passage through a flow restriction to receive fluid pressure therefrom wherein said fluid pressure acts against an area of said first make-up valve member to urge said first make-up valve member towards the position at which said first bypass passage is closed, and having means for causing said fluid pressure of said outlet passage to act directly on a smaller area of said first make-up valve member in a direction tending to cause said make-up valve member to open said first flow passage, and having means forming a second chamber receiving fluid pressure from said second outlet passage through a second flow restriction wherein said fluid pressure acts on an area of said second make-up valve member in a direction tending to cause said second make-up valve member to close said second bypass passage, and having means causing said pressure of said second outlet passage to act directly on a smaller area of said second make-up valve member in a direction tending to cause said second make up valve member to move to open said second bypass passage, and manually operated pilot valve for transmitting fluid pressure to said directional control valve to control said directional control valve, said pilot having a valve body with a pressurized fluid inlet and first and second outlet means communicated with opposite ends of said bore of said directional control valve and having a discharge passage and having a bore within said body, said pilot valve having a pilot valve spool movable in said bore between at least four positions including one position at which both of said outlet means are communicated with said discharge passage enabling said directional control valve spool to assume said Hold position thereof, and including a first motor operating position at which one of said outlet means is communicated with said inlet passage while the other of said outlet means is communicated with said discharge passage causing said directional control valve spool to assume said first of said motor operating positions thereof and including a second motor operating position at which the other of said outlet means is communicated with said inlet passage while said one outlet means is communicated with said discharge passage enabling said spool of said directional control valve to assume said second motor operating position thereof and further including a float position at which one of said outlet means is communicated with said inlet passage while the other of said outlet means is communicated with said discharge passage and while one of said chambers of said make-up valve means is also communicated with said discharge passage. 8. The combination defined in claim 7 further comprising a float valve connected between said pilot valve and both of said chambers of said make-up valve means, a spring biasing said float valve towards a first position at which said one chamber is blocked from said pilot valve while the other of said chambers is communicated therewith, and pilot means for causing said pressurized fluid to urge said float valve towards a second position at which said one chamber is communicated with said pilot valve while said other chamber is 

1. In a control system for a fluid motor (10) which has first and second motor ports (11, 12) wherein admission of pressurized fluid to said first motor port causes motor operation in one direction and admission of pressurized fluid to said second motor port causes motor operation in the opposite direction, the combination comprising: a directional control valve (17) having an inlet passage (59) for receiving pressurized fluid and having a discharge passage (74) and first and second outlet passages (69, 64) for connection to said first and second motor ports respectively and having a valve member (19) movable between a first motor operating position at which fluid from said inlet is transmitted to said first outlet while said second outlet is communicated with said discharge passage and a second motor operating position at which fluid from said inlet is transmitted to said second outlet while said first outlet is communicated with said discharge passage, and further having pilot means (51, 54) for shifting said valve member between said positions in response to fluid pilot pressure signals, make-up valve means disposed in a bypass passage (75) between at least one of said outlet passages of said directional control valve and said discharge passage and Having a first make-up valve member (78) movable to close said bypass passage and having means forming a first chamber (84) wherein fluid pressure derived from said one outlet passage acts against said first make-up valve member to urge said first make-up valve member towards the position at which said bypass passage is closed, and a pilot valve (25) having an inlet passage (56) for receiving pressurized fluid and a discharge passage (37) and an outlet means (33) communicated with said pilot means of said directional control valve for transmitting said fluid pilot pressure signals thereto to shift said valve member of said directional control valve, said pilot valve further having a valve member (31) movable between a plurality of motor operating positions for selectively transmitting said pilot pressure signals to said directional control valve, and being further movable to a float position, said pilot valve having venting means (36, 91, 95) for communicating said first chamber of said make-up valve means with said pilot valve means discharge passage upon said movement of said pilot valve member to said float position.
 2. The combination defined in claim 1 wherein said first make-up valve member has a flow orifice (85) therethrough to transmit said fluid pressure of said one outlet passage of said directional control valve to said first chamber.
 3. The combination defined in claim 1 wherein said first make-up valve member has a shoulder (87) thereon exposed to the fluid pressure of said one outlet passage.
 4. The combination defined in claim 1 wherein said pilot valve means has means (35, 49) for transmitting said pressure signals to said directional control valve pilot means to shift said valve member of said directional control valve to one of said motor operating positions thereof while said pilot valve member is at said float position thereof and said pilot valve means is communicating said first chamber of said directional control valve with said discharge passage of said pilot valve means.
 5. The combination defined in claim 1 wherein said make-up valve means further comprises means forming a second bypass flow passage (75a) between the other outlet passage (64) of said directional control valve and said discharge passage thereof, a second make-up valve member (79) movable to close said second bypass passage, means forming a second chamber (88) wherein fluid pressure derived from said other outlet passage acts against said second make-up valve member to urge said second make-up valve member to close said second bypass passage, a float valve (93) connected between said second chamber and said venting means, and means for opening said blocker valve in response to an unpressurized condition in said outlet means of said pilot valve means whereby said pilot valve means communicates said second chamber to said pilot valve discharge passage if said pilot valve member is at said float position and said fluid pilot pressure signal is absent.
 6. The combination defined in claim 5 wherein said float valve is connected between said venting means and both said first and second chambers and is spring biased towards a first position wherein said second chamber is communicated with said venting means while said first chamber is blocked therefrom, further comprising pilot means responsive to said fluid pilot pressure to urge said float valve to a second position at which said first chamber is communicated with said venting means while said second chamber is blocked therefrom.
 7. In a control system for a fluid motor having a first port to which pressurized fluid is supplied to drive said motor in a first direction and having a second port to which pressurized fluid is supplied to drive said motor in an opposite direction, the combination comprising: a directional control valve having a valve body with an inlet passage for receiving the pressurized fluid and first and second outlet passages for connection to said first and second motor ports respectiveLy and a discharge passage for discharging fluid, said valve body having a bore therein, said directional control valve having a first spool movable in said bore between a centered Hold position at which both of said outlet passages are blocked from said inlet passage and said discharge passage and having a first operating position at which said inlet passage is communicated with said first outlet passage while said second outlet passage is communicated to said discharge passage and having a second motor operating position at which said inlet passage is communicated with said second outlet passage while said first outlet passage is communicated with said discharge passage, and having spring means for urging said first spool towards said Hold position thereof; make-up valve means having means forming a first bypass passage between said first outlet passage and said discharge passage and having means forming a second bypass passage between said second outlet passage and said discharge passage and having first and second make-up valve members disposed in said first and second bypass passages respectively each being movable to close the associated bypass passage, and having means defining a first chamber communicated with said first outlet passage through a flow restriction to receive fluid pressure therefrom wherein said fluid pressure acts against an area of said first make-up valve member to urge said first make-up valve member towards the position at which said first bypass passage is closed, and having means for causing said fluid pressure of said outlet passage to act directly on a smaller area of said first make-up valve member in a direction tending to cause said make-up valve member to open said first flow passage, and having means forming a second chamber receiving fluid pressure from said second outlet passage through a second flow restriction wherein said fluid pressure acts on an area of said second make-up valve member in a direction tending to cause said second make-up valve member to close said second bypass passage, and having means causing said pressure of said second outlet passage to act directly on a smaller area of said second make-up valve member in a direction tending to cause said second make-up valve member to move to open said second bypass passage, and a manually operated pilot valve for transmitting fluid pressure to said directional control valve to control said directional control valve, said pilot having a valve body with a pressurized fluid inlet and first and second outlet means communicated with opposite ends of said bore of said directional control valve and having a discharge passage and having a bore within said body, said pilot valve having a pilot valve spool movable in said bore between at least four positions including one position at which both of said outlet means are communicated with said discharge passage enabling said directional control valve spool to assume said Hold position thereof, and including a first motor operating position at which one of said outlet means is communicated with said inlet passage while the other of said outlet means is communicated with said discharge passage causing said directional control valve spool to assume said first of said motor operating positions thereof and including a second motor operating position at which the other of said outlet means is communicated with said inlet passage while said one outlet means is communicated with said discharge passage enabling said spool of said directional control valve to assume said second motor operating position thereof and further including a float position at which one of said outlet means is communicated with said inlet passage while the other of said outlet means is communicated with said discharge passage and while one of said chambers of said make-up valve means is also communicated with said discharge passage.
 8. The combination defined in claim 7 further comprising a float valve connected between said pilot valve and both of said chambers of said make-up valve means, a spring biasing said float valve towards a first position at which said one chamber is blocked from said pilot valve while the other of said chambers is communicated therewith, and pilot means for causing said pressurized fluid to urge said float valve towards a second position at which said one chamber is communicated with said pilot valve while said other chamber is blocked therefrom, and means in said pilot valve for venting the one of said chambers which is connected therewith when said pilot valve is at said float position thereof. 